The present invention relates to a process employing segregation for producing aluminum or other metals in a very high state of purity in respect of eutectic elements.
With particular reference to aluminum, it is known that it is possible to reduce the proportion of elements which are referred to as eutectic elements such as copper, iron, magnesium, silicon and zinc in aluminum, when such elements are in a state of hypoeutectic concentration. To achieve this, the metal is melted in a container, and subjected to a segregation operation. In the course of this operation, cooling causes the production of crystals which are more pure in respect of eutectic elements than the liquid within which the crystals are formed. The crystals collect by gravity at the bottom of the container as they are formed and, by their being compacted, the result is a more or less compact, purified solid, the purity of which has a tendency to drop depending on the amount of mass crystallized. The operation is generally continued until only a small fraction of the mother liquor remains. Then, by different means, for example by means of sawing operations which are carried out after cooling, it is possible to separate the purified mass from the remaining mother liquor, or to even separate the purified mass into a number of fractions in different states of purity.
The efficiency of the purification operation is generally indicated by the value of the purification coefficient C.sub.O /C.sub.S, in which C.sub.S is the concentration of a given impurity in the pure product obtained, and C.sub.O is the concentration in respect of the same impurity in the metal used. The higher the purification coefficient, the more efficient is the treatment.
Processes based on that principle are disclosed in U.S. Pat. Nos. 3,303,019 and 4,221,590, and in French Pat. No. 1,594,154. These processes enjoy purification coefficients and yields which are higher or lower depending on the particular means employed.
Thus, in U.S. Pat. No. 3,303,019, the starting material used is a metal containing 280 ppm of iron and 420 ppm of silicon, and the process recovers therefrom 32% of the starting metal mass which fraction contains only 30 ppm of silicon and 10 ppm of iron. This corresponds to purification coefficients of 14 in regard to the silicon and 28 in regard to the iron, with a yield of 32%.
In U.S. Pat. No. 4,221,590, which is for an improvement in the above-mentioned process, the process may improve the yield in respect of purified metal having a silicon content of close to 100 ppm but, in contrast, the proportion of the same element scarcely falls below 20 ppm, for the purest fraction of aluminum, which represents only about 30% of the mass used.
As regards French Pat. No. 1,594,154, starting from a metal containing 320 ppm of silicon and 270 ppm of iron, the results obtained are respective amounts of 20 and 15 ppm, corresponding to purification coefficents of 16 and 18. These values are already very high if account is taken of the substantial yield achieved since it is of the order of 70%. Alternatively, taking a metal containing 620 ppm of silicon and 550 ppm of iron and with a yield of 50%, the process gives a metal containing only 40 and 10 ppm, respectively, of those elements, which is equivalent to purification coefficients of 15.5 and 55. The latter values are markedly higher than that set forth is U.S. Pat. No. 3,303,019, particularly when it is noted that the yield is 50% instead of 30%.
However, for some particular uses, it has been found necessary to provide aluminum in an even higher state of purity than that achieved with the above-described processes. Thus, for example, for producing medium-voltage and high-voltage electrolytic capacitors, manufacturers are increasingly having recourse to sheets of aluminum in which the proportion of Si and Fe must be only a few ppm, although the presence of certain elements such as copper may attain markedly higher levels of concentration, without thereby giving rise to problems.
In order to achieve such levels of purity, it is possible, when using the process of the French patent, to select the purest fractions, that is to say, those which are formed at the beginning of the operating procedure, but it is found that the recovery yield is then very low, being of the order for example of 10%, approximately, of the amount of metal used.
The attempt has then been made to apply the segregation process to a metal which has already been subjected to a first purification step, either by segregation or by another process such as refining by electrolysis in three layers.
However, serious difficulties were encountered in performing the operating procedure. It is found in fact that, with metals which are already very pure, the liquid has a tendency to solidify in masses of large volume on the walls of the settling apparatus and even of the crucible. That runs counter to the desired aim since French Pat. No. 1,594,154 states that, in order to achieve efficient purification, it is necessary to seek to achieve solidification of the material in the form of small crystals in order to limit the amount of mother liquor that the crystals retain between themselves. It is highly probable that this difficulty arises out of the fact that, the higher the level of purity of a metal, the smaller is the solidification gap (the temperature difference between the liquidus and the solidus of the equilibrium diagram).